1. Field of the Invention
The present invention relates to a differential amplifier with double output stages. More particularly, it relates to a differential amplifier which has double output stages. One of the output stages is a replica of another one. Such differential amplifier is a device that not only reduces the error of output signal, but also can avoid the disturbance of noise.
2. Background of the Invention
A conventional fully differential circuit usually used to the switched capacitor circuit, especially about switched capacitor filter. It must use the means of common mode feedback for the double terminals output of fully differential amplifier to control the common level of the double terminals output into a setting range of voltage, as usual, control the common level as the middle point of the setting range of voltage. In general, the common mode feedback circuit uses the structure of dynamic switched capacitor, and only one output stage used for providing the signal for the common mode feedback and the negative feedback circuits in the conventional balanced output operation amplifier. That is, the common mode feedback and negative feedback circuits use the same output stage, so that the charge injection noise and switching capacitor noise (KT/C) produced in the output signal which may result in operator error.
Referring to FIG. 1, is the schematic illustration of switching capacitor noise (KT/C). It's about the noise produced by the high frequency operation of switching mode element with series capacitor, and the effect for the noise of series capacitor could be a equivalent resistor shown in the schematic illustration of FIG. 1. FIG. 2 is the schematic illustration of charge injection noise. It shows that the switching mode element operated in the off state pushes the charge of the channel into the source and drain terminals, and produces an error voltage by the portion of charge into series capacitor. The error voltage may result in operator serious error which should be removed. FIG. 3 is the prior art that presents a switched capacitor filter utilizing a differential input and output circuit and method. The switched capacitor filter just uses only one output stage to provide output signal for a negative feedback and a common mode feedback, that is, the negative feedback circuit and the common mode feedback circuit are connected to the same output stage, and the output terminal will result in charge injection noise and switching capacitor noise which relieves the reliability of output signal.
As usual, reducing the switch size can resolve the problem of charge injection noise, but the switch size can't be reduced too much when the operation of circuit is rapidly modified. On the other hand, increasing the value of capacitor can resolve the problem of switching capacitor noise, but it will increase the chip area which result in more production cost. Moreover, using the two methods just reduce the two kind of noises but not filly eliminate the redundant noise. What is needed then is a circuit by which can efficiently eliminate the charge injection noise and switching capacitor noise.